As shown in FIG. 1, a typical switch-mode power supply (SMPS) includes primary side components 150 and secondary side components. Primary side, also referred to as “hot side” components comprise switch-mode controller 106, a switch-mode metal-oxide-semiconductor (MOSFET) 108, MOSFET heatsink 110, current sensing resistor 112, surge suppression capacitor 114, a transformer 116 having primary winding 120, secondary winding 118, rectifier diode 102, filter capacitor 104 and an opto-isolator 126. Secondary or “cold-side” components comprise secondary transformer windings 122 and 124, rectifier diodes 128 and 136 with their respective heatsinks 130 and 138 and filter capacitors 132 and 134. The entire switch-mode supply is powered from an unregulated voltage source 100. The controller 106 provides a drive signal VD to MOSFET 108 to produce current flow in primary winding 120 of transformer 116. Secondary winding 118 of transformer 116 provides a source of voltage, which when rectified and filtered by diode 102 and capacitor 104 respectively, provide supply voltage VDD to controller 106. Feedback signal VFB is developed from a rectified and filtered secondary supply +12V and fed back to controller 106 through opto-isolator 126, thus establishing a feedback loop to control the switching on and off of MOSFET 108. By comparison of feedback signal VFB to a reference value in controller 106 and variation of the conduction cycle of MOSFET 108 in response to differences between the feedback signal and the reference level, regulation of operating levels in the SMPS can be realized. Resistor 112 senses the primary current flowing in MOSFET 108 which serves as the current feedback signal to current-mode controller 106. Using current-mode control prevents excessive current to be drawn from the switch-mode supply under overload conditions. By rectifying a signal from transformer 116 secondary windings 122 and 124 by diodes 128 and 136 respectively, regulated output voltages +6.5V and +12V are developed and filtered by capacitors 132 and 134 respectively. Rectification of the signals developed across windings 122 and 124 may be accomplished by diodes in series with their respective windings between ground and the supply outputs. In this described typical SMPS, one of the diodes, 128, is placed with its cathode connected to the positive output of its particular supply, thus causing both anode and cathode of diode 128 to be remote from ground. In the exemplary +12V supply, diode 136 is placed such that its anode is connected to ground. In the type rectifiers described in this exemplary switch-mode supply, an often large source of inefficiency is the voltage drop across the rectifier diodes. In higher power supplies the inefficiency introduced by the voltage drop across the diodes can be significant, thus requiring heat sinking and possibly active measures such as forced air cooling.
In order to improve the rectifier efficiency, a transistor, usually a MOSFET may be used as a low voltage-drop switch to replace a diode. This technique is referred to as synchronous rectification. Synchronous rectification requires control of the drive to the synchronous rectifier to turn the MOSFET on or off during the appropriate portions of the signal being rectified. Integrated circuit controllers are often used to control conduction of the MOSFET. These integrated circuits, such as the ST Microelectronics STS-R3, or Anachip AP436 are moderately expensive and require an additional 4 to 8 external components. These ICs often include clock generation circuits and other sophisticated methods to determine on/off control of synchronous rectifier MOSFETS. The present invention involves a less complex control circuit that may use discrete components to provide a low-cost implementation of synchronous rectification in a switch-mode power supply.